Ammonia is well known as one of the promising substitute energy sources for fossil fuels, but it has some disadvantages such as low ignitability and low burning speed. Co-combustion with diesel fuel can compensate for its disadvantages and enable the application of the ammonia as a main fuel for internal combustion engines. In this study, the effects of ammonia/diesel mixing ratio and excess air ratio on combustion and emission characteristics have been investigated by internal combustion engine test and numerical approach. In the engine test, it was found that the ammonia/diesel mixing ratio and excess air ratio have a large effect on the heat release rate and emissions of nitrogen monoxide, nitrogen dioxide, unburned ammonia, and nitrous oxide. High ammonia mixing ratio leads to the aforementioned emissions, but these emissions were reduced in stoichiometric conditions compared to lean conditions. To investigate engine experimental results, the ammonia/n-heptane co-combustion emission φ-T map has been generated with zero-dimensional chemical reaction simulation. The φ-T map clarifies the low- temperature combustion leads to high unburned ammonia and nitrous oxide, hence stoichiometric combustion is preferable for ammonia combustion to reduce unburned ammonia and nitrous oxide preventing the increase of nitrogen monoxide and nitrogen dioxide emissions. Three-dimensional numerical simulation clarifies that unburned ammonia remains in the narrow space where diesel spray does not distribute such as crevice volume, and nitrous oxide remains on the surface of the unburned ammonia zone.
